1. Field of the Invention
This invention relates to a color cathode ray tube and, more particularly to a color cathode ray tube having a shadow mask and its manufacturing method.
2. Description of the Related Art
FIG. 1 shows a shadow mask type color cathode ray tube. Color cathode ray tube 1 comprises panel section 6, funnel section 8 and neck section 10, panel section 6 being composed of substantially rectangular faceplate 2 and skirt 4 extending from a peripheral edge of faceplate 2, funnel section 8 being bonded to skirt 4 of panel 6 and neck section 10 being continuous to funnel 8. The inside of cathode ray tube 1 is held airtight by panel 6, funnel 8 and neck 10. Electron gun assembly 11 for emitting electron beams is accommodated in neck 10. Deflection yoke 12 for generating a magnetic field is mounted on the outer surface of funnel 8 and neck 10. Phosphor screen 14 is formed on the inner surface of faceplate 2 of panel 6. Substantially rectangular shadow mask 16 is arranged in tube 1, facing phosphor screen 14 and spaced a specified distance from faceplate 2. Shadow mask 16 is made of a thin metal plate and has formed therein a multitude of slit apertures. Mask frame 18 is provided around shadow mask 16. A plurality of elastically deformable mask supporting members 20 are welded to mask frame 18. A plurality of stud pins 22, which engage with mask supporting members 20, are provided on a peripheral inner surface of skirt 14. Installed on the neck side of mask frame 18 is inner shield 24 to prevent the electron beams from electron gun 11 from being affected by the magnetic field.
In color cathode ray tube 1, the three electron beams emitted from electron gun assembly 11, after deflected by the magnetic field generated by deflection yoke, are converged near shadow mask 16. The converged electron beams land on phosphor screen 14 formed on panel 2. Phosphor screen 14 consists of three kinds of phosphor layers which are arranged in stripes alternately. Those phosphor stripes emit red, green and blue light rays when struck by the corresponding electron beams. Shadow mask 16 is provided such that the three electron beams are incident on the corresponding phosphor stripes.
FIG. 2 shows mask frame 18 mounted on skirt 4 of panel 6. Three mask supporting members 20a and one mask supporting member 20b are attached near the mid-positions of the four sides of mask frame 18, respectively. Each mask support member 20a is composed of bimetal 26 attached to mask frame 18 and metal plate 28 welded to bimetal 26. Mask supporting member 20b is composed of bimetal 26, metal plate 28 and washer 30 being in contact with stud pin 22. Mask supporting members 20a and 20b are engaged with stud pins 22, respectively.
Mask supporting member 20a is shown in FIG. 3A and mask supporting member 20b is shown in FIG. 3B. In each mask supporting member 20a, metal plate 28 has a hole that catches stud pin 22, the diameter of this hole is equal to a diameter of stud pin 22 of a portion which is to be fitted into the hole for maintaining the specified distance between shadow mask 16 and phosphor screen 14. Also in mask supporting member 20b, metal plate 28 has a hole that catches stud pin 22. Washer 30 has a hole and the diameter of this hole is equal to a diameter of stud pin 22 of a portion which is to be fitted into the hole for maintaining the specified distance between shadow mask 16 and phosphor screen 14, and smaller than the hole of plate 28 that catches stud pin 22. That is, the diameter of the hole of plate 28, which catches stud pin 22 is larger than a diameter of stud pin 22 of a portion which is to be fitted into the hole for maintaining the specified distance between shadow mask 16 and phosphor screen 14. Shadow mask 16 is held in tube 1 by the following method. First, the holes of metal plates 28 of only three supporting members 20a are fitted onto stud pins 22 and thereby mask frame 18 is temporary held up in panel 6. At mask supporting member 20b, stud pin 22 is put into the hole of metal plate 28, but this mask supporting member 20b does not contribute to holding of the shadow mask, in other words, maintaining of the specified distance between shadow mask 16 and phosphor screen 14 since the hole diameter is larger than a diameter of stud pin 22 of a portion which is to be fitted into the hole for maintaining the specified distance between shadow mask 16 and phosphor screen 14. Then, washer 30 of mask supporting member 20b is inserted between stud pin 22 and metal plate 28. Since the hole diameter of mask supporting member 20b is larger than the diameter of the nearest stud pin, mask supporting member 20b is movable. Therefore, by moving that peripheral portion of shadow mask 16 which is around mask supporting member 20b, the position of shadow mask 16 is adjusted and then, washer 30 is welded to metal plate 28 of mask supporting member 20b. Thus, shadow mask 16 is mounted in panel 6 correctly.
However, if shadow mask 16 is mounted to panel 6 by the above method, the area where the correct position of shadow mask 16 with respect to the faceplate is secured by three mask support members 20a is very small. In other words, the area of the overlapped portion which the triangle formed on a plane lying at right angles with the tube axis by projecting thereon a triangle having three apexes at three stud pins 22 overlaps with the substantial quadrangle formed on a plane lying at right angles with the tube axis by projecting shadow mask 16 thereon, is about 1/4 of the area of the substantial quadrangle. In consequence, if the distance between panel 6 and shadow mask 16 is measured after shadow mask 16 is mounted to panel 16, there will be a wide area that does not comply with the specification. If a color cathode ray tube is manufactured with such a substandard area present, the result will be a color picture tube of low color purity. Hence, the mass-production aptitude of such color cathode tubes is low. To take an example, if the above-mentioned method of mounting the shadow mask is applied to over 40-inch color cathode ray tubes for high-quality color television sets which are in growing demand, with the screen aspect ratio of 16:9, which is a new standard, the shadow masks are liable to be deformed. This is because in those color cathode ray tubes, the long sides of the mask frames are longer than those of ordinary mask frames. In other words, if the distance between a panel and a shadow mask is adjusted by using supporting members and the shadow mask is mounted to the panel just as in the above method, the shadow mask is likely to suffer torsional deformation. Therefore, it is extremely difficult to make a color cathode ray tube where the distance from the panel complies with the standard across the whole area of the shadow mask.